1. Field of the Invention
The present invention relates to an electro-optical apparatus, an electronic apparatus and the method for driving the electro-optical apparatus and, more particularly, to an electro-optical apparatus that precharges a data line prior to the writing of an image signal, the method for driving the electro-optical apparatus, and an electronic apparatus that incorporates the electro-optical apparatus.
2. Description of Related Art
One example of electro-optical apparatus is a liquid-crystal apparatus employing an active-matrix driving method based on thin-film transistors (hereinafter referred to as TFTs). Japanese Unexamined Patent Publication No. 2-204718 discloses a liquid-crystal apparatus that includes --liquid-crystal pixels disposed in a matrix form, thin-film transistors for driving its respective liquid-crystal pixels, rows of scanning lines and columns of data lines. In this liquid-crystal apparatus, the scanning lines, data lines and pixel electrodes corresponding to each cross of scanning lines with data lines are arranged on a TFT array substrate. Besides these elements, the TFT array substrate includes a diversity of peripheral devices, specifically peripheral circuits such as a sampling circuit, a precharge circuit, a scanning line driving circuit, a data line driving circuit, and a check circuit.
The scanning line driving circuit selects one row of liquid-crystal pixels every horizontal scanning period by scanning a plurality of scanning lines on a line-at-a-time basis. The data line driving circuit successively samples image signals to be supplied to each data line during one horizontal scanning period, and writes the image signal on the one row of liquid-crystal pixels selected by the scanning line driving circuit, on a point-at-a-time basis. To assist the writing of the image signal to the liquid-crystal pixels, the precharge circuit performs a precharge operation to write a predetermined potential to the one row of liquid-crystal pixels prior to the writing of the image signal by the data line driving circuit.
More specifically, the precharge circuit is a circuit that supplies the data line with a precharge signal (preliminary charging signal) prior to the timing at which the data line driving circuit supplies the image signal to the data line via the sampling circuit, with the purpose of enhancing a contrast ratio, stabilizing the potential level of the data line, and reducing an on-screen line non-uniformity. The precharge operation helps reduce the load of the data line driving circuit when it writes the image signal onto the data line.
Particularly when a so-called line inversion driving method commonly performed to AC-drive the liquid crystal, namely, a method in which the polarity of the voltage of the data line is inverted every predetermined period, is employed, the supply amount of charge required to write the image signal onto the data line can be substantially reduced by writing beforehand the precharge signal onto the data line in the precharge operation. One example of such a precharge circuit is disclosed, for example, in Japanese Unexamined Patent Publication No. 7-295520.
In the conventional liquid-crystal apparatus, a TFT in the precharge circuit is connected to each precharge signal line for receiving the precharge signal. The precharge signal line is connected to a large capacitance component and a large resistance component arising from the capacitance between the gate and source/drain in the TFT and the resistance of the wiring for the data line.
The precharge operation is carried out by concurrently writing a predetermined potential to the data lines and one row of liquid-crystal pixels. When the precharge signal is written in this way, a large current instantaneously flows through the precharge signal line. The wiring resistance of the precharge signal line and the capacitance component attached to the signal line increase as the wiring length of the precharge signal line becomes long in step with an increasing size of a liquid-crystal panel. As the wiring resistance and the capacitance component increase, the precharge signal is more subject to a delay. Furthermore, the larger the panel size, the more delay the precharge signal is subject to.
When a circuit arrangement with the precharge signal supplied from one end of the precharge signal line is employed, the precharge signal is more distorted in its waveform due to the signal delay and the voltage of the signal tends to drop, as the precharge signal travels further from the input end of the line. As a result, the supply amount of charge written on the data line during a predetermined precharge period becomes different depending on the distance between the input end for the precharge signal and the location of each data line.
The problem of the signal delay arises not only in the precharge signal line, but also in a precharge circuit driving signal line, namely, a signal line for supplying a precharge circuit driving signal that determines the timing of supplying the precharge signal. The precharge circuit driving signal line is the signal line connected to the gate of each TFT in the precharge circuit. The writing of the precharge signal is executed for a predetermined period, by supplying a gate signal having a predetermined pulse width to the precharge circuit driving signal line.
In the circuit in which the gate signal is supplied from one end of the precharge circuit driving signal line, the gate signal is deformed in its waveform due to the signal delay on the line and the voltage of the signal tends to drop, as the gate signal travels further from the input end of the precharge circuit driving signal line. In an area where the gate signal fails to rise sufficiently in voltage, the period during which the TFT in the precharge circuit is turned on gets shorter, and the data line is not sufficiently charged up to the precharge potential. As a result, the supply amount of charge at the data line with the precharge signal supplied becomes different depending on the layout location of each data line.
When the amount of charge which is written onto the data line by the precharge signal during the precharge period is different depending on the layout position of each data line, a potential difference takes place at the data lines subsequent to the supply of the respective image signal even if the data lines are supplied with the image signal of the same potential. The potential difference from data line to data line causes non-uniformity in luminance (transmittance ratio) in the screen of a liquid-crystal apparatus.
The luminance non-uniformity becomes problematic, particularly, in a three-panel type projector. Known as one of electro-optical apparatuses is a three-panel type projector which, employing three identically constructed liquid-crystal panels, synthesizes the three primary color lights modulated through the three liquid-crystal panels to present a color image. The projector synthesizes two primary color images which are optically inverted (hereinafter referred to as "inverted images") after being transmitted through liquid-crystal panels and one primary color image which is not optically inverted (hereinafter referred to as "non-inverted image") after being transmitted through a liquid-crystal panel, thereby producing a color image.
When the transmittance ratio of the liquid-crystal panel is different between one pixel close to and another pixel far from the input terminal side for the precharge signal, the non-uniformity in transmittance ratio in the inverted image and the non-uniformity in transmittance ratio in the non-inverted image appear at different locations. A synthesized image, based on these images, presents therefore a difference in transmittance ratio which each of the three primary color images suffers. Each liquid-crystal panel modulates its particular color light, and if there appears, on the synthesized image, a difference in transmittance ratio in each of the three primary color images, no correct color reproduction is performed, and a chrominance non-uniformity occurs between a left-hand side portion and a right-hand side portion of the synthesized image.
As described above, in the conventional liquid-crystal apparatus, the luminance non-uniformity, chrominance non-uniformity or the like are created by a diversity of causes, thereby degrading the image. The vision of humans is particularly sensitive to a difference in color, and the chrominance non-uniformity is of a particular concern in a large-size and high-definition color liquid-crystal projector employing a plurality of liquid-crystal panels.
The signal delay of the precharge signal takes place not only in a large-size panel but also in a high-definition panel. Specifically, when the precharge signal is supplied to the data line from the precharge signal line, a large charging and discharging current instantaneously flows through an opposite electrode of each liquid-crystal pixel (a common electrode that is diametrically opposed to a corresponding pixel electrode of each pixel with a liquid-crystal layer interposed between the opposite electrode and the pixel electrode), a capacitance electrode (an electrode that is diametrically opposed to the pixel electrode of each pixel with an insulating film interposed between the capacitance electrode and the pixel electrode, forming a storage capacitor), and other elements.
The higher the definition of the panel, the narrower the width of wiring of the panel, and the higher the wiring resistance of the opposite electrode and the capacitance electrode becomes. A large potential difference takes place in the wiring in the liquid-crystal apparatus when a large charging and discharging current flows. The potential difference decays with time at a time constant determined by the wiring resistance and the parasitic capacitance of the wiring section.
A short horizontal scanning period in the high-definition panel makes it difficult to set up a period for the voltage difference to decay therewithin. Because of its shorter signal period, the high-definition panel results in a relatively high inductance component in the wiring in the panel, possibly creating an oscillation while the charging and discharging current flows, and thereby suffers difficulty decaying the potential difference in the electrodes. For this reason, when the panel is a high-definition one, problems such as a degradation in contrast attributed to variations of precharged charge arise. Furthermore, when there are above-described variations in the amount of precharged charge, the device is subject to erratic operations because of variations in the potential of the opposite electrode, the potential of the capacitance electrode, and the GND potential of the circuit, and because of radiated noise that is created by the charging and discharging current to the electrodes or the like of these potentials.